Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 156

6 Hydrophobically associated networks

6.1 Introduction

In this chapter we are concerned with molecules which have a dual nature, with one part
soluble in water (hydrophilic) and the other a non-polar part which is expelled from water
and aqueous systems (hydrophobic); such molecules are sometimes known as amphiphilic.
Molecules of this kind are forced to adopt unique orientations with respect to the aqueous
medium and sometimes form organized structures. Such molecules were first recognized to

play an important role in living matter, speci cally in its organization, of which cell

membranes are the best examples. The formation of these membranes is spontaneous
and depends only on the fact that their constituent molecules are amphiphilic.

6.2 The hydrophobic effect

6.2.1 Origin of the hydrophobic effect

Hydrophobic substances dissolve readily in many non-polar solvents, but are hardly
soluble in water. By contrast, hydrophilic molecules have a strong af

nity for water, and
tend to repel one another strongly; they contain electronegative atoms able to establish
hydrogen bonds with water, and include alcohols, sugars, polar (non-ionic) groups,
anionic (e.g. carboxylate or sulphonate) and cationic (trimethyl ammonium) groups
and zwitterions (Israelachvili, 1992 ). The functional groups are believed to have a
disordering effect and disrupt the local structure of liquid water, while hydrophobic
molecules tend to increase the ordering of water molecules. Amphiphilic molecules such
as surfactants (surface-active agents) have a particular architecture: one end (called the
head) contains a hydrophilic group, while the rest of the molecule (the tail) is hydro-
phobic, usually with a long hydrocarbon chain.
Surfactants are known mainly for adsorbing on to surfaces (e.g. between liquid and gas
phases) or interfaces (e.g. between two immiscible liquids). Soaps are a good example of
surfactants; these are usually sodium or potassium salts of straight-chain fatty acids
containing between 8 and 19 carbon atoms. Other characteristics of surfactants include
the capacity, in solution, to form clusters of colloidal size known as micelles.
It was recognized in the early 1920s (McBain and Salmon, 1920 ) that the reversible
formation of micellar aggregates in soap solutions was due to association between